The present invention relates to a liquid crystal display device having at least one, and preferably a plurality of pixel elements.
In a standard liquid crystal display device, the pixel element is formed in a liquid crystal layer (normally common to the pixel elements) extending in a plane, and there is at least one polarizing arrangement parallel to the plane of the liquid crystal layer. By applying electrical signals to the liquid crystal layer using suitable electrodes, it is possible to vary the angle of polarization of polarized light passing through the liquid crystal layer, and thereby to vary the optical transmissivity of a liquid crystal display device with the change in polarization relative to the polarizing arrangement. Normally, in such a liquid crystal display device, the polarizing arrangement is formed by two polarizing plates, one on each side of the liquid crystal layer, but it is also possible to provide an arrangement with a single polarizing plate on one side of the liquid crystal layer and a reflective element on the other side of the liquid crystal element.
In standard liquid crystal display devices, electrical fields are generated by electrodes arranged perpendicular to the plane of the liquid crystal layer. Therefore, if the change in the liquid crystal layer due to the electric fields is to be visible, the size of those electrodes needs to be large; and, therefore, it is necessary to use transparent electrodes. Furthermore, at least two layers are normally needed between the transparent electrodes on each side of the liquid crystal layer and the liquid crystal layer itself. One such layer forms an orientation layer for the liquid crystal layer, but a further insulating layer is then needed between the orientation layer and the transparent electrode.
In International Patent Application No. PCT WO91/10936, a liquid crystal display device was disclosed in which electrical signals were applied to the Liquid crystal layer so as to generate electric fields having components in a direction parallel to the plane of the liquid crystal layer. Such parallel field components cause reorientation of the molecules of the liquid crystal layer, thereby varying the optical transmissivity of the liquid crystal display device.
In PCT WO91/10936, it was proposed that the electrodes for applying such field were, for each pixel element, in the form of combs, the teeth of the comb formed by one electrode extending into the spaces between the teeth of the comb formed by the other electrode. The teeth of each electrode were electrically connected in common, and a voltage was applied between the electrodes.
JP-B-63-21907 (1988) also disclosed a liquid crystal display device in which electrical signals were applied to the liquid crystal layer so as to generate electric fields having components in a direction parallel to the plane of the liquid crystal layer. As in PCT WO91/10936, the electrodes for applying such fields were, for each pixel element, in the form of combs. Use of comb-shaped electrodes was also disclosed in U.S. Pat. No. 4,345,249.
In each of these known arrangements, each pixel element has first and second electrodes of comb shape, with the teeth of one comb extending between the teeth of the other comb. Voltages are then applied to the electrodes by a suitable control circuit. It is important to note that the teeth of the comb-shaped electrodes are not electrically independent, so that the size of the pixel is determined by the size of the comb-shaped electrode.
The principles of operation of such devices, with comb shaped electrodes, is also discussed in an article entitled xe2x80x9cField Effects In Nematic Liquid Crystals Obtained With Interdigital Electrodesxe2x80x9d by R. A. Soref in the Journal of Applied Physics, pages 5466 to 5468, vol. 45, no. 12 (December 1974), and in an article entitled xe2x80x9cInterdigital Twisted-Nematic-Displaysxe2x80x9d by R. A. Soref, published in the Proceedings of the IEEE, pages 1710 to 1711 (December 1974).
In the standard liquid crystal display devices discussed above, it is necessary to use transparent electrodes, which are formed on facing surfaces of two substrates. However, in order to form such transparent electrodes, it is necessary to use a vacuum manufacturing operation, such as sputtering, and thus the cost of manufacture of such standard liquid crystal display devices is high. Furthermore, it has been found that such transparent electrodes have vertical geometrical irregularities, of the order of several tens of nanameters, which prevents precise manufacture of active devices, such as thin film transistors needed to control the signals to the electrodes. Also, it has been found that parts of such transparent electrodes may become detached, to cause point or line defects. Thus, it has proved difficult to manufacture liquid crystal devices both reliably and cheaply.
Such conventional liquid crystal display devices also have disadvantages in terms of picture quality. The problem of vertical geometrical irregularities in the transparent electrodes has been mentioned above, but similar irregularities around the controlling transistors may result in orientation failure domains being formed, requiring a light shielding film to cover such transistor devices, reducing the light utilization efficiency of the liquid crystal device. Also, such conventional liquid crystal display devices have the disadvantage that there is a significant change in brightness when the visual angle is changed, and reversion of some gradation levels can occur in a half-term display, at some view angles.
Although the use of comb shaped electrodes, such as previously discussed, involves the need for transparent electrodes, further problems have been found. While the use of such comb-shaped electrodes offers theoretical advantages, those are limited by practical consideration which have to be taken into account when the comb-shaped electrodes are used. If the teeth of such comb-shaped electrodes have a width of 1 to 2 micrometers, satisfactory practical operation can be achieved. However, it is extremely difficult to form such fine teeth over a large substrate without defects. Thus, in practice, the aperture factor of the liquid crystal display device is reduced, because of the need to provide relatively wide electrode teeth. There is thus a trade-off between aperture factor and production yield, which is undesirable.
Therefore, the present invention seeks to provide a liquid crystal display device which is more suitable for mass production than the known liquid crystal display devices discussed above. The present invention has several aspects.
In the liquid crystal display device according to the present invention there are features which are common to all the aspects. The device has a liquid crystal layer, and at least one polarizing arrangement, which is normally provided in the form of a pair of polarizing plates disposed on opposite sides of the liquid crystal layer. The device has at least one, and normally a plurality, of pixel elements and there are electrodes which receive electrical signals for controlling the optical transmissivity of light through the device. As in e.g. JP-B-63-21907 (1988) discussed above, the electrical signals are applied such that electrical fields are generated in the liquid crystal layer with components parallel to the plane of the liquid crystal layer. The various aspects of the present invention, which will be discussed below, then relate to the electrode arrangement of a pixel element(s) and also to the materials and optical arrangements of the materials of the liquid crystal display device.
In a first aspect of the present invention, each pixel element has a pixel electrode extending in a first direction within the pixel, and there are also signal wiring electrodes extending in the same direction over several of the pixel elements. There are also common electrodes extending in that first direction over more than one of the pixel elements.
There may be a pair of pixel electrodes for each pixel element, with the signal wiring electrodes then extending between a pair of pixel electrodes at each pixel element. There is then a pair of common electrodes, with the pair of pixel electrodes being disposed therebetween, so that electrical fields are generated in opposite directions for each pixel element.
Preferably, all the electrodes are on the same side of the liquid crystal layer. Arrangements are also possible, however, in which the common electrodes are on the opposite side of the liquid crystal layer formed the other electrodes. In either case, if there is insulating material between the common electrode and the pixel electrode for each pixel element, a capacitive device may be formed therebetween.
In practice, it is possible for the common electrodes to be provided in common for two adjacent pixel elements, by interacting with pixel electrodes on opposite sides of each common electrode.
In a second aspect of the present invention, each pixel can be considered to have a elongate transistor element extending in a first direction, that elongate transistor element having at least one elongate electrode. There is also at least one elongate common electrode extending in the same direction as the elongate transistor element. In the second aspect, an insulating film separates the at least one elongate electrode of the elongate transistor element and the at least one common electrode.
In a third aspect of the present invention, each pixel has again an elongate transistor element extending in one direction, and at least one elongate common electrode extending in the same direction. The elongate transistor element has at least one elongate electrode, and there is an insulating film extending over and in direct contact with that at least one elongate electrode. That insulating film is also in direct contact with the liquid crystal layer. Preferably, that insulating film is an organic polymer.
In a fourth aspect of the present invention, each pixel can be considered to have a transistor element with a pair of first electrodes (pixel electrodes), a signal electrode between the first electrodes, and a gate electrode. The first and second electrodes extend in a common direction, as do a pair of common electrodes. A transistor element is then (in plan) disposed between the pair of common electrodes. As has previously been mentioned, the common electrodes and the transistor element may be on the same side of the liquid crystal layer, or they may be on opposite sides.
In a fifth aspect, for any one pixel element, each one of said pairs of common electrodes thereof forms a corresponding one of said pair of common electrodes for pixel elements adjacent to said any one pixel element.
The five aspects of the present invention discussed above all relate to an electrode arrangement of the liquid crystal display device. The aspects of the invention to be discussed below relate to the optical arrangement and materials of the liquid crystal display device.
In a sixth aspect of the present invention, the angles between components of electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at opposite surfaces of the liquid crystal layer are the same, and the product of the thickness of the liquid crystal layer and the refractive index anisotropy of the liquid crystal layer is between 0.21 xcexcm and 0.36 xcexcm.
In a seventh aspect of the present invention, the absolute value of the difference between the angles between components of electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at opposite surfaces of the liquid crystal layer is not less than 80xc2x0 and not greater than 100xc2x0, and the product of the thickness of the liquid crystal layer and the refractive index anisotropy of the liquid crystal layer is between 0.4 xcexcm and 0.6 xcexcm.
In an eighth aspect of the present invention, the dielectric constant anisotropy of the liquid crystal layer is positive, and the absolute value of the angle between components of the electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at the surface of the liquid crystal layer is less than 90xc2x0, but not less than 45xc2x0.
In a ninth aspect of the present invention, the dielectric constant anisotropy of the liquid crystal layer is negative, and the absolute value of the angle between components of the electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at the surface of the liquid crystal layer is greater than 0xc2x0, but not greater than 45xc2x0.
In a tenth aspect of the present invention, the dielectric constant anisotropy of the liquid crystal layer is positive, and the value of the difference between: i) the angle between components of the electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at the surface of the liquid crystal layer; and ii) the angle of the polarization axis of said at least one polarizing plate and said components of the electric fields in a direction parallel to the plane of said liquid crystal layer, is 3xc2x0 to 15xc2x0.
In an eleventh aspect of the present; invention, the dielectric constant anisotropy of the liquid crystal layer is negative, and the value of the difference between i) the angle of the polarization axis of said at least one polarizing plate and said components of the electric fields in a direction parallel to the plane of said liquid crystal layer, and ii) the angle between components of the electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at the surface of the liquid crystal layer, is 3xc2x0 to 15xc2x0.
In a twelfth aspect of the present invention, the is direction of orientation of molecules of said liquid crystal layer at a surface of said liquid crystal layer parallel to the plane of said liquid crystal layer and said surface is not more than 4xc2x0.
Although various aspects of the present invention have been discussed above, a liquid crystal display device embodying the present invention may incorporate combinations of such aspects. Depending on the resulting combination, the present invention provides advantages in the manufacture and/or operation of a liquid crystal display device, and these advantages will be discussed in more detail later.